1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. Specifically, the present invention relates to a semiconductor device in which a transistor is formed from a polycrystalline semiconductor crystallized by laser light irradiation and to a method of manufacturing the semiconductor device. The invention also relates to a semiconductor device which uses the above transistor in its pixel and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, portable equipment such as PDAs, cellular phones, and notebook computers have grown popular. Such equipment has flat panel displays mounted thereto. Flat panel displays employed often are STN liquid crystal displays, amorphous silicon TFT (amorphous silicon thin film transistor) liquid crystal displays, and the like. However, lately these displays have been taken over by low temperature polysilicon (polycrystalline silicon) TFT liquid crystal displays in which driving circuits are built on glass substrates. In near future, even they will be replaced by light emitting displays (e.g., EL displays) that use low temperature polysilicon TFTs as major flat panel displays mounted to portable equipment.
An example of a pixel circuit in a light emitting display is shown in FIG. 26, and the operation thereof will be described briefly. The pixel circuit shown in FIG. 26 controls a current flowing into a light emitting element 2602 (hereinafter referred to as light emission current) by controlling Vgs (gate-source voltage) of a driving transistor 2601. The value of the light emission current and the luminance of the light emitting element are in proportion to each other. Therefore, the luminance of the light emitting element can be controlled by controlling the light emission current.
Light emitting elements can be formed of a wide range of materials including organic materials, inorganic materials, and bulk materials. Among them, a typical light emitting element is an organic light emitting diode (OLED), which is mainly composed of organic materials. A light emitting element is structured to have an anode, a cathode, and a light emitting layer which is sandwiched between the anode and the cathode. A light emitting layer is formed of one or more materials chosen from the above materials. Luminescence provided by light emitting layers is divided into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). The present invention is applicable to a case in which one of the two types of light emission is used as well as a case in which both types of light emission is used.
FIG. 27A is a circuit diagram of a circuit composed of the light emitting element 2602 and the driving transistor 2601. FIG. 27B shows the relation between Vgs (gate-source voltage) of the driving transistor 2601 and the light emission current. In FIG. 27B, there are two curves indicating that the characteristic of one driving transistor 2601 is different from the characteristic of another driving transistor 2601. When the driving transistor 2601 is fluctuated in characteristic as shown in FIG. 27B, the light emission current is also fluctuated even though Vgs stays the same level. Accordingly, fluctuation in characteristic of the driving transistor 2601 has to be avoided in order to display an image in accurate gray scales. It is impossible to display an image in correct gray scales if mobility, threshold, and other characteristics of the driving transistor 2601 are fluctuated.
On the other hand, low temperature polysilicon TFTs are distinctively high-performance TFTs formed on glass substrates. In order to enhance the performance of a TFT, the crystallinity of a semiconductor (specifically a channel formation region) in the transistor has to be improved.
A widely used method for improving the crystallinity of a semiconductor is to irradiate an amorphous state semiconductor with laser light and crystallize the semiconductor (polycrystallization or making amorphous silicon into polysilicon). According to this method, only a portion irradiated with laser light receives high energy and therefore unnecessarily subjecting the entire substrate to high temperature can be avoided. A TFT formed by this method is called a low temperature polysilicon TFT.
A TFT formed by a method in which a semiconductor layer is crystallized by thermal annealing is called a high temperature polysilicon TFT.
In many cases, a laser used in forming a low temperature polysilicon TFT is an excimer laser and its laser light is shaped into a linear shape before irradiating a glass substrate. The entire glass substrate is irradiated with the laser light by running the linear laser light over the substrate.
FIG. 28 is a schematic diagram of laser light irradiation. A linear laser 2801 is scanned (irradiated) in Direction x. In FIG. 28, the linear laser irradiates parallel to the source driver and scanning by the laser is moved in parallel to the gate driver. Such laser light irradiation method is described in, for example, JP 2756530 B.
The description given next is about how pixels are arranged into a matrix pattern in a pixel portion. In a pixel region 2802 of FIG. 28, plural pixels are arranged to form a matrix pattern. If the pixel portion is for displaying a monochrome image, the pixels are placed at regular intervals in the longitudinal direction and the lateral direction both. On the other hand, if the pixel portion is for displaying a color image, there are various ways to arrange R pixels, G pixels, and B pixels.
Methods of arranging pixels for R color, pixels for G color, and pixels for B color are described with reference to FIGS. 29A and 29B. FIG. 29A shows a longitudinal stripe arrangement in which pixels for the same color are lined up longitudinally. FIG. 29B shows a delta arrangement in which pixels on one row and pixels on the next row are staggered by half a sub-pixel.
In the longitudinal stripe arrangement, the length of a stripe of R color pixels in the lateral direction is one third the length of this stripe in the longitudinal direction. The same applies to a stripe of G color pixels and a stripe of B color pixels. If an R color pixel, a G color pixel, and a B color pixel together make one pixel, the length of this pixel in the longitudinal direction is equal to the length of the pixel in the lateral direction to form a square shape. In other words, a pixel pitch N of one pixel in the longitudinal direction is equal to a pixel pitch N of the one pixel in the lateral direction. In the delta arrangement, the length of an R color pixel (sub-pixel) in the longitudinal direction is the same as its length in the lateral direction. The same applies to a G color pixel (sub-pixel) and a B color pixel (sub-pixel). In other words, an R color pixel (sub-pixel), a G color pixel (sub-pixel), and a B color pixel (sub-pixel) each have a square shape.
As has been described, a semiconductor crystallized by linear laser light irradiation is used in a low temperature polysilicon TFT. Here, referring to the laser light intensity distribution (FIG. 30), a description is given on the operation of a linear laser when a semiconductor is irradiated with laser light by running the linear laser over the semiconductor.
First, one point in a semiconductor is irradiated with a linear laser. The laser light intensity distribution at this stage often forms a hill shape as shown in FIG. 30. An example thereof is Gaussian distribution. Thereafter, the laser light irradiation position is moved in Direction x by a laser scan pitch M to irradiate the semiconductor with the linear laser again. Then the laser light irradiation position is once more moved in Direction x to proceed laser light irradiation. The same operation is repeated to irradiate the entire glass substrate with the laser.
In this laser light irradiation, some regions of the semiconductor are irradiated with the laser light many times whereas some other regions are irradiated with the laser light only a few times depending on where they are located in Direction x (the laser scanning direction) as shown in FIG. 30. In short, the laser light irradiation number varies from one region of the semiconductor to another.
Furthermore, the intensity of laser light emitted from a laser is not constant but is fluctuated. This means that regions of a semiconductor cannot be irradiated uniformly with laser light even if they receive the same number of laser light irradiation.
When the number of laser light irradiation and the laser light intensity are varied from one region of a semiconductor to another, the crystallinity of the semiconductor crystallized by the laser is also fluctuated. Transistors formed from a semiconductor that has uneven crystal state are fluctuated in characteristic.
If transistors that are fluctuated in characteristic are used to manufacture a light emitting display, the driving transistor 2601 of one pixel and the driving transistor 2601 of another pixel exhibit different characteristics. Such light emitting display is incapable of displaying an image in accurate gray scales.
FIG. 31 shows the light emitting display which is displaying an uneven image because of the influence of fluctuation in characteristic of the driving transistor 2601. This uneven image is due to varying laser light irradiation number and laser light intensity, which differ from one point to another in Direction x (the laser scanning direction) of the semiconductor. As a result, streaks parallel to Direction y appear showing laser light irradiation tracks. This type of image unevenness is hereinafter referred to as laser fringes.